This invention relates to network protectors which connect an electric power distribution network to multiple feeder buses. Such network protectors have a protective relay to disconnect the network from a feeder bus upon detection of reverse power flow out of the network to the feeder bus. More particularly, it relates to diagnostics for detecting abnormal operating conditions in the network protector and preferably to a system which reports such abnormal operating conditions to a remote station.
A low voltage secondary power distribution network consists of interlaced loops or grids supplied by two or more sources of power so that the loss of any one source will not result in an interruption of power. Such networks provide the highest level of reliability possible with conventional power distribution and are normally used to supply high-density load areas such as a section of a city, a large building or an industrial site. Each source is a medium voltage multiphase feeder supplying the network with three phase power and consisting of a multiphase bus, a switch and a transformer. A network protector connects the multiphase feeder bus to the multiphase network and consists of a circuit breaker and a control relay. The control relay senses the transformer and network voltages and line currents, and executes algorithms to initiate breaker tripping or closing action. Trip determination is based on detecting reverse power flow, that is, power flow from the network to the feeder.
Traditionally, the control relays have been electromechanical devices which trip the circuit breaker open upon detection of power flow in the reverse direction. Such relays have been provided with a recloser which closes the circuit breaker following a trip when conditions are favorable for forward current flow. The electromechanical network protector relays are being replaced by electronic relays. One type of electronic network protector relay mimics the action of the electromechanical relay by calculating power flow. Another type of electronic network protector relay uses sequence voltages and currents to determine direction of current flow for making tripping decisions. Sequence analysis upon which such relays are based generates three vector sets to represent a three-phase voltage or current: a positive sequence vector, a negative sequence vector, and a zero sequence vector. U.S. Pat. No. 3,947,728 discloses a sequence based network protector relay which uses the positive sequence current and positive sequence voltage vectors to make the trip decision. More recently, a digital sequence based network protector relay has been utilized. This relay digitizes the voltages and currents and uses the digitized values to calculate the sequence components. This relay also performs metering functions and can include a communication system for transmitting the metering data to a remote station for storage and analysis.
It is common for the network protectors to be located under ground where they are protected from moisture and other adverse conditions in a vault. If the circuit breaker cannot be maintained closed to provide power to the network, it is necessary to send out a repair crew. Often, the cause or existence of a problem is not known until the repair crew can make an inspection. This can delay correcting the problem. Some network protectors have indicators indicating the status of the device but this does not necessarily provide an indication of the cause of the problem.
There is a need for an improved network protector including one with diagnostics which can indicate the nature of a problem.
There is a further need for such an improved network protector which provides an indication of the problem at a remote location so that a crew dispatched to the faulty network protector does not have to perform an on-site analysis and can come prepared to correct the known problem.
These needs and others are satisfied by the invention which is directed to a network protector with diagnostic apparatus and to diagnostic apparatus for use with a network protector. More particularly, the diagnostic apparatus includes means measuring selected parameters, processing means processing the selected parameters to detect abnormal operating conditions in the network protector, and means generating an by output indicating the abnormal operating condition. Preferably, the abnormal operating condition is transmitted to a remote station by a communication system. Some of the diagnostic checks which can be performed include measurement of the electrical resistance of the separable contacts in the multiphase power circuit of the network protector. This test is performed using a first voltage detector provided in the control relay which measures the voltage across the separable contacts, and a multiphase current detector that measures current passing through the separable contacts in each phase. Processing means within the diagnostic apparatus then calculates the electrical resistance of the separable contacts from the voltage readings across the separable contacts and the current flowing through the contacts. If this electrical resistance is above a predetermined value, an abnormal contact resistance indication is generated. This abnormal operating condition indication can be presented at the network protector and/or can be communicated to the remote station over the communication system.
The multiphase current generated by the current detector can be used to detect a blown fuse in a phase of the power circuit through the circuit breaker. To this end, the processing means generates an indication of a blown fuse condition if the current through a phase of the multiphase power circuit of the network protector is zero while current is detected in at least one other phase.
Where the circuit breaker of the network protector includes an operating mechanism which is actuated by a trip solenoid to open the separable contacts, the diagnostic apparatus includes a second voltage detector which monitors the voltage available to operate the trip solenoid through monitoring the voltage across a set of normally open trip contacts. If this voltage is below a predetermined value indicative of insufficient voltage to trip the circuit breaker, a low trip voltage indication is generated for optional transmission to the remote station.
For a network protector which includes a charging motor for charging the operating mechanism, another voltage detector monitors the voltage across normally open close contacts which energize the motor when closed. As in the case of the trip solenoid, a low close voltage abnormal operating condition indication is generated if the voltage available across the normally open contacts is insufficient to initiate charging of the operating mechanism. When these normally open close contacts are closed to energize the motor, the voltage across the motor is measured. The diagnostic apparatus generates a low motor voltage indication when the voltage across the motor is below a certain value. In a unique arrangement, a common voltage detector monitors the voltage across the normally open contacts when these contacts are open and automatically switches to measuring the voltage across the motor when the normally open contacts closed.